The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At the mechanical resonance, the device appears to be an electronic resonator; hence, the device can act as a filter. The mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the device for a given phase velocity of sound in the material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and few microns in thickness.
At the heart of Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) is a thin sputtered piezoelectric film having a thickness on the order of one to two microns. Electrodes on top and bottom sandwich the piezoelectric acting as electrical leads to provide an electric field through the piezoelectric. The piezoelectric, in turn, converts a fraction of the electric field into a mechanical field. A time varying "stress/strain" field will form in response to a time-varying applied electric field.
To act as a resonator, the sandwiched piezoelectric film must be suspended in air to provide the air/crystal interface that traps the sound waves within the film. The device is normally fabricated on the surface of a substrate by depositing a bottom electrode, the PZ layer, and then the top electrode. Hence, an air/crystal interface is already present on the topside of the device. A second air/crystal interface must be provided on the bottom side of the device. There are several prior art approaches for obtaining this second air/crystal interface.
The first approach involves etching away the wafer that forms the substrate. If the substrate is silicon, the silicon is etched away from the backside using hot KOH. This leaves the resonator constructed on the front side of the wafer supported by its edges. The holes made through such a wafer render the wafer very delicate and highly susceptible to breakage. Furthermore, using wet etches such as KOH with their 54.7 degree etch slope limits the ultimate density and thus the yield of FBAR/SBAR filters on a wafer. For example, devices with lateral dimensions of approximately 150 .mu.m by 150 .mu.m build on a standard 530 .mu.m thick silicon wafer, require a backside etch hole roughly 450 .mu.m by 450 .mu.m. Hence, only about 1/9.sup.th of the wafer can be productively utilized.
The second prior art method for providing an air/crystal interface under the device is to create an air-bridge type FBAR/SBAR device. Typically, a sacrificial layer is first laid down, and the device is then fabricated on top of this sacrificial layer. At or near the end of the process the sacrificial layer is removed. Since all of the processing is done on the front side, this approach does not suffer from having two-sided alignment and large area backside holes. However, this approach is not without inherent difficulties. First, the method is difficult to practice on large devices. Typically, the sacrificial layer is thermally grown SiO.sub.2 which is removed using HF. The etch rate is of the order of 1000 to 3000 A/minute. To etch under device areas that are on the order of 150 .mu.m by 150 .mu.m or larger, an etch time greater than 500 minutes is needed. In addition to being excessively long, the exposure of the metal electrodes to the etchant for periods in excess of 30 minutes leads to the delamination of the metal electrodes from the piezoelectric layer.
The third prior art approach is referred to as the solidly mounted resonator (SMR), since there are no air gaps under the devices. The large acoustic impedance at the bottom of the device is created by using an acoustic Bragg reflector. The Bragg reflector is made up of layers of alternating high and low acoustic impedance materials. Each thickness is fixed to be at the quarter wavelength of the resonant frequency. With sufficient layers, the effective impedance at the piezoelectric/electrode interface is much higher than the device acoustic impedance, thus trapping the sound waves effectively in the piezoelectric.
While this approach avoids the problems discussed above in creating a free standing membrane, it has a number of problems. The choice of materials used in the Bragg reflector is limited, since metals cannot be used for these layers because the metal layers would form parasitic capacitors that degrade the electrical performance of the filters. The degree of difference in the acoustic impedance of layers made from the available dielectric materials is not large. Accordingly, more layers are needed. This complicates the fabrication process as the stress on each layer must be well controlled. After many layers, the device is not conducive to integration with other active elements, since making vias through 10 to 14 layers is difficult. Furthermore, the devices reported to date have significantly lower effective coupling coefficients than devices having air bridges. As a result, filters based on SMRs exhibit reduced effective bandwidths compared to air bridge devices.
Broadly, it is the object of the present invention to provide an improved FBAR/SBAR device.
It is a further object of the present invention to provide an FBAR/SBAR device that does not require back etching of the substrate.
It is a still further object of the present invention to provide an FBAR/SBAR device that does not require excessively long etch times to create an air gap under the device.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.